1. Field of the Invention
The present invention generally relates to communications systems, and more particularly to an improved system and method for concatenating Reed-Solomon and trellis encoders.
2. Discussion of the Related Art
In recent years, there has been significant research and development in the area of communications. Significantly, there is an ever increasing demand for increased data communication rates for the purpose of decreasing data transmission time. However, an increase in the rate of the data used to modulate the carrier frequency typically results in increased bandwidth requirements.
In an effort to increase the data rates without sacrificing the available bandwidth, a number of modulation schemes together with sophisticated coding techniques have been developed. The coding of data in a transmitter, prior to transmission, and decoding of data in a receiver, after transmission, is generally referred to as "channel coding." The basic motivation for channel coding has been to reduce the frequency of errors in the output information bit stream for a give signal to noise ration, or conversely, to increase the transmission rates at which information can be transmitted with a given probability of error (P.sub.e). Stated another way, bandwidth efficient line codes can be used to provide higher bit rates in a given bandwidth, or, alternatively, they can also be used to reduce the required bandwidth for a given bit rate.
For example, Quadrature Amplitude Modulation (QAM) employs both amplitude and phase modulation in order to encode more data within a given frequency bandwidth. Carrierless amplitude modulation and phase modulation (CAP) is an encoding method that utilizes a two-dimensional multilevel modulation scheme. As is known, these high level modulation schemes are very sensitive to channel impairments. That is, the information encoded by means of such techniques is often lost during transmission due to noise, Raleigh fading and other factors which are introduced over the communication medium.
In order to compensate for the increased sensitivity of these high level modulation schemes, various forward error detection and error correction coding techniques are employed. Reed-Solomon and trellis are two commonly used and well known forward error correction coding techniques. As is well known, these techniques are often used together, since they complement one another. In this regard, trellis encoders, which are implemented to protect data against channel impairments, are known to be susceptible to producing burst errors, because trellis encoders make decoding decisions over several symbols. As a result, if the path is incorrect, several symbols along that path may be incorrectly decoded. The purpose of Reed-Solomon encoding is to compensate for burst errors made by the trellis encoder.
In a common communications system, these techniques are combined by directing an information word (a series of information bits of a predetermined length to be transmitted) of length k symbols is input to a Reed-Solomon encoder. In a manner that is well known, the Reed-Solomon encoder generates n-k parity symbols, for an output of length n symbols. This output is then time displaced by an interleaver before being directed to the trellis encoder for further encoding. The trellis encoder, which generally operates only on a few of the coded bits output from the Reed-Solomon encoder, generates further redundancy in the transmitted signal.
U.S. Pat. No. 3,988,677 to Fletcher et al. and U.S. Pat. No. 5,659,578 to Alamouti et al. examples of two systems that concatenate Reed-Solomon and trellis encoders. A common trait among the systems of the prior art, as is illustrated in both of the foregoing patented systems, is that all of the information bits from the data source are encoded by the Reed Solomon encoder. As the length of the data word increases, the size of the interleaver, interposed between the Reed-Solomon encoder and trellis encoder, increases as well.